LNG is usually primarily liquefied methane containing varying quantities of ethane, propane and butanes with trace quantities of pentanes and heavier hydrocarbon components. Usually the LNG is low in aromatic hydrocarbons and non-hydrocarbons such as H2O, N2, CO2, H2S and other sulphur compounds, and the like, as these compounds have usually been removed at least partially before liquefying the natural gas stream, which is then stored or transported in liquid form. For the purpose of this description, ‘hydrocarbon stream’, ‘LNG’ or ‘natural gas’ should not be construed to be limited to a certain composition, but rather be seen as a liquid stream in general, in particular a hydrocarbon containing stream.
It is desirable to liquefy natural gas for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form, because it occupies a smaller volume and does not need to be stored at high pressures.
In order to regasify the LNG stream it is usually pressurized and vaporised. If desired a selected amount of e.g. N2 is added to obtain natural gas having a desired gas quality, e.g. a selected heating value (i.e. energy content when the gas is burned), according to gas specifications or the requirements of a consumer. Alternatively or additionally, the heating value of the natural gas may be adjusted by removing or adding a desired amount of ethane and/or heavier hydrocarbons from the natural gas.
An example of a method for the regasification or vaporization of LNG of the so-called ‘intermediate fluid type’ is disclosed in US 2005/0274126 A1. More particularly, US 2005/0274126 describes a method and apparatus for vaporizing cryogenic fluids such as LNG in which an intermediate heat transfer fluid is first heated across a heat transfer surface with ambient air and then the heat transfer surface provide heat to vaporize the cryogenic fluid.
A problem of the known method of regasifying or vaporizing LNG is that relatively high capital expenses (CAPEX) have to be made.